The present invention relates to a foamable polyester resin composition. More particularly, the invention relates to a foamable polyester resin composition which has excellent low-temperature processability and gives a highly expanded foamed molding excellent in strength and surface properties.
Polyester resins give moldings excellent in physical properties such as transparency, mechanical properties, gas barrier properties, and heat resistance and in chemical properties such as solvent resistance, acid resistance, and alkali resistance, and are hence extensively used as packaging materials, e.g., bottles, sheets, and films and in other various fields. Recently, foam molding is attracting attention as a technique for reducing the weight of polyester resins, improving the heat-insulating properties of the resins, and reducing the cost of moldings. There is a strong desire in the market for a highly expanded polyester resin molding.
In a generally known technique for the foam molding of a polyester resin, a chain-branching agent is used in combination with a blowing agent.
It is known that use of, for example, a carboxylic acid dianhydride as the chain-branching agent is effective in heightening the molecular weight of a molten polyester resin or improving the melt viscosity thereof and thereby enabling foam molding to be conducted in a high expansion ratio.
For example, Japanese Patents 1,921,348 and 2,126,744 disclose that a foamed molding having an improved expansion ratio can be obtained by adding a compound having two or more acid anhydride groups in the molecule, such as pyromellitic dianhydride, as a chain-branching agent to the polyester resin to be foamed.
However, since pyromellitic dianhydride has a melting point as high as 280xc2x0 C., it produces its effect only when the resin is heated and molded at a temperature of from 270 to 290xc2x0 C. When the molding was actually conducted in a low-temperature range of up to 210xc2x0 C., almost no effect was observed.
The present invention is intended to attain a high expansion ratio even when foam molding is conducted in a low-temperature range of from 150 to 210xc2x0 C., by using a (meth)acrylic ester polymer without using any of those chain-branching agents. The invention hence differs from the technique described above.
Furthermore, Japanese Patent Laid-Open No. 2000-136301 discloses that a foamed molding having an expansion ratio of from 1.6 to 3.0 is obtained from a composition prepared by adding an acrylic resin to an alloyed resin which is a blend of a polycarbonate with another aromatic-ring-containing thermoplastic resin containing a polyester resin.
The invention enables a further improvement in expansibility by using a (meth)acrylic ester polymer having a specific value of specific viscosity produced by (co)polymerizing one or more monomer mixtures having a specific composition. Furthermore, the invention mitigates the decrease in strength in foamed moldings attributable to an increase in expansion ratio, by adding a filler. The invention hence differs from the technique of the related art described above.